The natural vibration behaviour of structural elements of an aircraft or spacecraft determines the structural dynamic behaviour of the whole aircraft or spacecraft, or at least the affected structural elements. For example, a structural component vibrating at its natural frequency, for example an engine, also induces vibrations in an adjoining structural element, for example an engine casing. The adjoining structural elements which are coupled together excite one another in such a way that undesirable deformations of the structural elements can result therefrom. Furthermore, the natural vibration behaviour of a structural element also determines its so-called aeroelastic behaviour or the aeroelastic behaviour of the whole aircraft or spacecraft. The interaction of the structural element with an air flow is referred to as aeroelastics, and the behaviour of the structural element in the air flow is referred to as aeroelastic behaviour of the structural element. Alongside structural dynamic effects, the structural element is also subjected to elastic deformations caused by air flows. The elastic deformations resulting both from the structural dynamic properties and from the aeroelastic properties of a structural element can, for example, lead to undesirable vibrations in the structural element. This in turn can cause an increased noise development, a partial loss of function of the structural element such as in the case of juddering of a control surface, or even a premature material fatigue.
Various strategies for influencing the vibration behaviour of the structural element are known to the applicant from experience. Additional masses are often fixed to an affected structural element, whereby, for example, the natural vibration frequencies of the structural element can be influenced. With regard to the vibration behaviour of the structural element, this approach does indeed lead to good results. However, the disadvantage then arises that in the case of operating an aircraft or spacecraft, unnecessary i.e. so-called dead or non-structural masses must be moved. This leads inter alia adversely to an increased fuel consumption because of the additional weight.
Alternatively, it is possible to modify the rigidity of the affected structural element in such a way that the natural vibration behaviour is influenced. This can be achieved, for example, by modifying the geometry or topology of the structural element or, in the case of fibre composite constructions, by varying the fibre orientations and/or the fibre layer structure accordingly. Disadvantageously for this approach, however, it has been found that the geometry of the structural element, which is optimised with regard to lightweight construction and aerodynamic behaviour, must be modified. Even modifying the fibre orientation and/or layer structure means an undesirable change with regard to the achievable mechanical properties.
Furthermore, it is possible to use passive or active damper elements. The use of damper elements means, however, an increased number of components in the structural element. This means, disadvantageously, an additional weight and increases, as a further disadvantage, the complexity of the structural element.
DE 698 05 302 T2 describes, for example, a structural element for an aircraft or spacecraft, the rigidity of which can be actively modified to control the vibration behaviour of the structural element. To this end, a cross section of the structural element and thus its rigidity is modified by means of a piezo element integrated in the structural element. The piezo element, which is arranged in a recess of the structural element, is converted for this purpose from an unstretched to a stretched state, whereby the piezo element touches two opposite walls of the recess only in a stretched state and thus transfers forces from one wall to the other wall. The rigidity of the structural element is thus modified and its aeroelastic properties can thus be actively influenced. However, this approach necessitates the use of additional components, meaning, alongside an additional weight, an increased complexity and probability of failure of the structural element, which is an additional disadvantage.
It is therefore one object of the present invention to provide an improved structural element for an aircraft or spacecraft which eliminates the aforementioned disadvantages.